As a solder joint structure formed by soldering a metal member and a joint member using a solder material, for example, the power module as shown in Patent Documents 1 and 2 can be exemplified.
The power module is provided with a power module substrate formed by soldering a metal plate (for example, aluminum (Al)) which is to become a circuit layer on one surface of a ceramic substrate, and a power element (semiconductor device) mounted on one surface of the circuit layer.
In addition, on the other side of the power module substrate, in order to dissipate heat from the power element (semiconductor device), a radiator plate or cooler is provided at times as a heat sink.
In the power module described above, the circuit layer and the power element (semiconductor device) are joined via a solder material. That is, it is configured as a solder joint structure in which the circuit layer is formed of a metal member (aluminum member) and the power element (semiconductor device) is formed of a joint member.
In addition, the power module substrate and the heat sink are also joined through a solder material in some cases.
It should be noted that in those cases where the circuit layer or the like is formed of an aluminum member made of aluminum or aluminum alloy, since an oxide film of aluminum is formed on the surface of the circuit layer, it is impossible to carry out soldering, as it is, in a favorable manner.
Further, when the circuit layer or the like is formed of a copper member made of copper or a copper alloy, the melted solder material and copper react and the components of the solder material infiltrates into the inside of the copper member of the circuit layer or the like to alter the characteristics, which is a problem.
Accordingly, conventionally, a Ni plating film has been formed as a base layer on the surface of the circuit layer by electroless plating or the like, and the semiconductor device has been joined by disposing a solder material on the Ni plating film.
Here, in the power module described above, a power cycle and heat cycle are loaded at the time of use thereof. When the power cycle and heat cycle are loaded in the power module, there was a possibility that the stress due to the difference in coefficients of thermal expansion of the ceramic substrate and circuit layer (aluminum) acts on the joint interface between the ceramic substrate and the circuit layer, thereby reducing the joint reliability. Accordingly, conventionally, the circuit layer is formed with a metal member (aluminum) having a relatively small deformation resistance such as 4N aluminum with a purity of 99.99% or more, so that the thermal stress described above is absorbed by deformation of the circuit layer, thereby improving the joint reliability.
In addition, in Patent Document 3, a conductive composition (solder base layer forming paste) capable of forming a conductive joining layer (solder base layer) to conduct a circuit layer made of aluminum or aluminum alloy and a solder layer without providing a Ni plating film has been described, and a structure in which the above circuit layer and a joint member are joined through a glass layer, an Ag layer, and a solder layer has been disclosed.